The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Some embodiments relate to oil and gas production, more specifically, to the methods of producing polymer emulsion for downhole operations and mixing degradable (hydrolysable) polymer emulsion with the treatment fluid.
Polyesters (i.e. polylactic acid (PLA) or polyglycolic acid (PGA)) as well as their mixtures and copolymers, polyorthoesters, polyanhydrides, polycarbonates or other degradable polymers (DP) are important materials known as environmentally safe plastics widely used for the production of polymer films, powders and fibers. For downhole operations, these materials are used in the form of fibers or powders as a proppant carrier or another formation rock permeability modifying material. Another form of these polymers are water emulsions that can be easily injected into the well.
Degradable polymers (DP) are preferable if the bond between the polymer and the formation rock should be reversible or temporary. Injection of various DPs into wells solves a number of problems such as permeability control, proppant and fine particulate migration, desired proppant location and proppant settling rate control in the well, the task of formation rock consolidation and bridging, acid treatment of formation, etc.
Various commercial degradable polymers are known (1-2) and many of which are specially designed for downhole applications. The preferred DP class are polyesters produced by polymerization of hydroxycarbonic acid, e.g. lactic acid polyesters known as polylactic acid (PLA), glycolic acid polyesters known as polyglycolic acid (PGA), 3-hydroxybutyric acid polyesters, 2-hydroxyvalerianic acid polyesters, e-caprolactone polyesters, hydroxyaminoacid esterification polyesters such as serine, trionine and tyrosine, soy protein type polypeptides, other polyesters and polyamides, polyanhydrides, polycarbonates, polyols (polyatomic alcohols), other downhole degradable polymers and copolymers derived from abovementioned monomers.
Several methods of DP delivery to the well and into the formations are known. In Patent Application US200502272613, incorporated by reference, it is suggested to deliver DP in the form of grains or viscous fluids. This required extra equipment (a storage grid in the DP injected well), high temperatures (about or above the polymer melting point) and injecting an inhomogeneous system which may hinder reliable operation of other equipment. DP delivery in protective shells (U.S. Pat. No. 7,080,688, incorporated by reference) allows producing acid monomers as the polymer degrades in the hydraulic fracture. Polymer delivery in the form of fibers was described in Patent Application WO2006137002A1, incorporated by reference.
Patent Application US20070032386, incorporated by reference, discloses a method of delivering degradable polymers (PLA or PGA) in the form of suspended small particles. The particle size of the degradable polymer is small enough to penetrate into the formation rock pores and produce a cake as a result of controlled polymer pyrolysis. This near wellbore treatment material was called Internal Filter Cake/Matrix Breaker. After hydrolysis into monomer acids, this material works as an efficient acid reactant for gel degradation.
However, the known DP delivery methods in solid state (suspended particles) require a preparation stage to provide polymer particles of the desired size. The rate and completeness of solid (suspended) polymer hydrolysis is difficult to control, because polymer swelling and hydrolysis are limited by diffusion of formation fluid into the degradable polymer particles.
This invention characterizes the preparation and delivery of DP into the well in the form of emulsion. Emulsions are two-phase (two liquids) systems wherein one phase is dispersed in the main media in the form of small particles (droplets). In the case considered herein, the dispersed fluid contains preliminarily dissolved polymer material. Because many emulsions have physical properties (viscosity, pipe friction, etc.) similar to those of usual treatment fluids, polymer emulsions can be added to wells using standard pumps and tubing.
Although degradable polymer emulsions are well known in industry (e.g. US2005/0058712, “Aqueous dispersion of at least one biodegradable polymer”, incorporated by reference), little is known about the preparation of degradable polymer (DP) emulsions for downhole operations. Besides, the controllable decay of polymer emulsion remained beyond the coverage of this prior art.
One of downhole applications was characterized in Patent Application PCT/IB2007/054455 (Willberg et al.), incorporated by reference. That invention relates to solid acid polymers that produce active acids under specific conditions. More particularly, that invention relates to polyester materials that degrade to acids and also covers the use of those monomer acids for underground formation treatment.
It has been claimed that a solid acid precursor (e.g. PLA or PGA) can be dissolved in a non-toxic and inexpensive organic solvent and emulsify the solution in a disperser. The method was illustrated by using PLA and other acid precursors for the acid treatment of formations wherein acid was formed by polymer hydrolysis. In-the-field operations are easy with water emulsions of degradable polymers: the emulsion can be prepared and stored in tanks as a regular liquid and then delivered to the well using standard pumps and pipes.
Further DP degradation in the target environment depends strongly on downhole temperature and media pH.
However, many downhole operations require more rapid and controlled modification of physical properties of the treatment fluid (polymer droplet size, softened polymer settlement in underground formation pores, cohesion with solid particles added to the hydraulic fracturing fluid and macroscopic distribution of the polymer phase). This can be achieved by destructing the PLA water emulsion back to immiscible components (water and the organic phase).
Patent Application US2007/0298977A1 (Obtaining Degradable Particles and Related Methods, Halliburton), incorporated by reference, describes methods of obtaining hydrolysable particles near wellhead and methods of using hydrolysable polymer particles in the downhole environment. Under that method, the emulsion is used as an intermediate step in the production of a suspension of solid polymer particles suitable for delivery into the well and for the treatment of the formation rock.
The known method comprises the steps of preparing a mixture of said hydrolysable polymer and a first solvent, adding the ready mixture to a second solvent with a sufficient shear stress for obtaining emulsion in a wellhead plant. Then sufficient quantity of the first solvent is removed from the dispersed phase to form a phase of dispersed particles in a continuous phase (water).
At an intermediate step of preparing a polymer-in-water emulsion, the process efficiency can be controlled by adding surfactants.
Disadvantages of that technical solution are that the method requires the use of chlorine containing solvents (the first solvent for PLA taken as a hydrolysable polymer), the stability of suspended particles is difficult to control after mixture preparation in a wellhead blender and the resultant small solid particles may not have the physical properties as are required for a specific application.
It is therefore suggested to use emulsions suitable for the generation of sticky settleable (and hydrolysable) polymer in a well or in a well piping which is convenient for many downhole applications and to control the polymer settling process using a set of stabilizers and destabilizers. The method does not require any toxic or inflammable substances.
The object of some embodiments is to provide new methods of applying amorphous degradable polymers on well surface portions.